In the sound recording industry, there is substantial interest in restoring old recordings of vocal performances as well as instrumental and orchestral recordings.
One approach to improving the acoustic quality of old sound recordings, as described in U.S. Pat. No. 4,186,280, involves selective amplification or filtration of the acoustic signal mixture, in accordance with the frequency ranges corresponding to the identified sound sources or parts (solo voices, solo instrument, predominant instrumental group and the like) on the record. But suppression or amplification of certain frequencies does not accurately reproduce the original sound.
The motion of a cutting tool 10 in a plastic media encodes sound on phonograph records, as shown in FIG. 1. In mono recordings the cutting tool 10 is moved back and forth in one direction. For stereo recordings, the cutting tool is moved in two orthogonal directions, encoding two channels of stereo signal in the velocity of two orthogonal groove walls (6 and 8 as shown in FIG. 1). The two walls (6 and 8) form a “V” groove in the record with each wall forming an angle of approximately 45 degrees with the plane of the record. After a master is cut, multiple copies are made using industry standard pressing techniques.
Standard techniques for playback of records involve measuring the rate of change of the groove position (its “velocity”), typically by a stylus 4 (shown in FIG. 2) which rides in the groove 2 of the record (either the copy or the original cut master). When a record is damaged due to wear, dirt or mishandling, a groove wall 8 may be damaged (exemplified by FIG. 2) so that the playback stylus 4 cannot move in the same manner as was originally encoded on the record. In any system, velocity measurements are quite noisy when small position errors are introduced. In other words, small positional deviations (caused by imperfections or damage) result in large instantaneous velocities. This is a reason why a record degrades so noticeably when it is worn. Small imperfections in the groove are amplified in the velocity signal produced by the stylus. These small positional defects cause sound degradation and often “clicks” or “pops”.
Conventionally, record companies attempting to reconstruct old recordings will play a record using different size styli in an effort to read different parts of the groove and interleave the different “plays” of the record. By choosing different size styli, the groove is being tracked at different heights. The goal is to find a location in the groove that is relatively pristine. However, this method has drawbacks because the damage to the groove is often excessive and varies significantly at different locations.
Thus, there continues to be a need to reconstruct sound recordings from a worn or damaged record by means of a method which is true to the originally intended sound recording, whereby damage throughout the record does not create significant unintended noise.